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doi:10.2204/iodp.proc.342.110.2014

Biostratigraphy

Coring at Site U1409 recovered a 200 m thick sequence of Pleistocene to lower Paleocene nannofossil ooze and nannofossil clay, with foraminifers and radiolarians. Nannofossils, planktonic foraminifers, and benthic foraminifers are present through most of the succession, although all microfossil groups are absent through a short interval between the Pleistocene and Oligocene. Radiolarians are only present in the uppermost Pleistocene and the lower middle Eocene through the upper Paleocene. Thin Pleistocene and Oligocene intervals overlie a middle Eocene through lower Paleocene succession with significant hiatuses between the lower Pleistocene and upper Oligocene (22 m.y. duration) and lower Oligocene and middle Eocene (8.3 m.y. duration). A short hiatus or condensed interval is also identified at the Paleocene/Eocene boundary. The Oligocene is highly condensed and may contain significant hiatuses. Sedimentation rates are 0.68–1.31 cm/k.y. through the middle Eocene, 0.51–1.44 cm/k.y. through the lower Eocene, and ~0.47–1.80 cm/k.y. through the Paleocene.

Benthic foraminifers are generally rare (the “present” category) throughout the recovered succession, with the exception of parts of the lower Oligocene and Sample 342-U1409A-22X-CC (179.32 mbsf), where they are dominant. Benthic foraminifer preservation is good to very good through most of the recovered Eocene sequence. Moderate to poor preservation occurs in the Oligocene and Paleocene.

An integrated calcareous and siliceous microfossil biozonation is shown in Figure F21. Datum and zonal determinations from nannofossils, planktonic foraminifers, and radiolarians are in close agreement. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F30. A summary of calcareous and siliceous microfossil abundances and preservation is given in Figure F22.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher samples from Hole U1409A and additional working section half samples from Holes U1409A–U1409C. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Calcareous nannofossil occurrence data are shown in Table T4. Note that the distribution charts are based on shipboard study only and are, therefore, biased toward age-diagnostic species.

At Site U1409, the preservation of calcareous nannofossils is generally good to exceptional in the middle Eocene and moderate to good in the Oligocene, lower Eocene, and upper Paleocene. The uppermost sediment from Hole U1409A contain abundant nannofossils indicative of Pleistocene Zones NN20/21–NN16 marked by the top of Pseudoemiliania lacunosa in Sample 342-U1409A-2H-CC (10.64 mbsf) and the top of Discoaster pentaradiatus in Sample 3H-4, 75 cm (15.85 mbsf). Samples 3H-4, 75 cm (15.85 mbsf), through 3H-CC (20.60 mbsf) are noncalcareous and do not contain nannofossils. The short interval from Sample 4H-CC to 5H-2, 75 cm (30.08–31.85 mbsf), is assigned to Oligocene Zones NP23–NP21 based on the top of Reticulofenestra umbilicus (Sample 342-U1409A-5H-2, 75 cm; 30.08 mbsf) and Coccolithus formosus (Sample 342-U1409B-4H-7, 27 cm; 36.57 mbsf). The identification of Zone NP21 in Sample 342-U1409B-4H-CC, 6.5 cm (37.08 mbsf), and Zone NP17 in Sample 4H-CC, 15 cm (37.17 mbsf), indicates the presence of a hiatus of ~6 m.y., representing the upper Eocene to lowermost Oligocene.

Samples 342-U1409A-5H-4, 75 cm, through 26X-CC (34.85–200.03 mbsf) are assigned to middle Eocene to lower Paleocene nannofossil Zones NP17–NP4. The majority of primary zonal marker species are present and listed in Table T3.

The identification of Subzone NP14a in Sample 342-U1409A-14H-3, 110 cm (199.22 mbsf), and Zone NP12 in Sample 14H-5, 110 cm (122.20 mbsf), indicates the presence of a short hiatus (~1.5 m.y.). In addition, the samples taken in the lowermost part of the green clay-rich drift sediment contain nannofossils indicative of Zone NP14 alongside reworked nannofossils from lower in the Eocene (Zone NP12 equivalent). The same nannofossils, Chiphragmolithus spp., were also seen at comparable levels (i.e., at the onset of drift sedimentation, approximately Zone NP14) at Sites U1408 and U1406.

The Paleocene–Eocene transition is identified by the presence of Zone NP10 in Sample 342-U1409A-20X-3, 48 cm (154.28 mbsf); Subzone NP9b in Sample 20X-3, 56 cm (154.36 mbsf); and Subzone NP9a in Sample 20X-3, 77 cm (154.57 mbsf). The very short Subzone NP9b interval (<0.29 m) suggests condensed sedimentation and likely a short hiatus, but the presence of the nannofossil excursion taxon Discoaster araneus indicates that PETM-equivalent time is represented around Sample 20X-3, 56 cm (154.36 mbsf).

The sequence from Sample 342-U1409A-20X-3, 77 cm, to 26X-CC (154.57–200.03 mbsf) is assigned to Subzone NP9a–Zone NP4 based on the base of Discoaster multiradiatus (Sample 20X-5, 114 cm; 157.95 mbsf), the base of Discoaster mohleri (Sample 22X-4, 78 cm;175.28 mbsf),‘and the base of Heliolithus kleinpellii (Sample 24X-2, 80 cm; 191.50 mbsf). The lowermost three cores, 342-U1409A-24X through 26X, contain poorly preserved nannofossil assemblages, but the absence of Fasciculithus and presence of Toweius pertusus suggests correlation with Zone NP4.

Radiolarians

Radiolarian biostratigraphy is based on analysis of all core catcher samples from Hole U1409A. No samples from Hole U1409B or U1409C were examined. Radiolarians are present in the uppermost part of Hole U1409A (Cores 342-U1409A-1H and 2H) but are either absent or rare in the underlying Pleistocene–middle Eocene interval downhole to Core 11H (96.4 mbsf). Below this level, radiolarians are very abundant in the uppermost lower Eocene, rare but well preserved in the underlying lower Eocene, and generally abundant and well preserved in the Paleocene. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5, and the radiolarian distribution is shown in Tables T6 and T7. Note that the distribution charts are based on shipboard study only and are, therefore, biased toward age-diagnostic species.

Cores 1H through 2H (1.02–10.64 mbsf) contain a Pleistocene–Holocene radiolarian assemblage assigned to Zone RN17 based on the absence of Stylatractus universus.

Samples 342-U1409A-3H-CC through 11H-CC (20.6–96.4 mbsf) are either barren of radiolarians or contain only poorly preserved spumellarians that cannot be assigned to species.

In contrast, radiolarians are very abundant and well preserved in the core catcher samples from Cores 12H and 13H. Sample 12H-CC (105.96 mbsf) is assigned to Zone RP11 based on the presence of the primary index species Dictyomitra mongolfieri and the absence of Eusyringium lagena, the primary index species for Zone RP12. Sample 13H-CC (115.96 mbsf) is assigned to Zone RP9 based on the presence of the primary index species Theocorys plesioanaclasta and the absence of a key index species for Zone RP10, Lithochytris vespertilio. Zone RP10 was not identified but may be present within Core 13H because the zone has a duration of only 0.59 m.y.

Samples 14H-CC through 17X-CC (124.91–131.33 mbsf) contain common radiolarians of moderate preservation. This interval is assigned to Zone RP8 based on the presence of the primary index species Buryella clinata and the absence of T. plesioanaclasta and Theocorys anaclasta, both of which have bases in Zone RP9.

Samples 18X-CC through 19X-CC (138.99–141.21 mbsf) contain only rare and poorly preserved radiolarians.

Samples 20X-CC through 21X-CC (160.59–162.71 mbsf) contain abundant well-preserved radiolarians. Sample 20X-CC (160.59 mbsf) is assigned to Zone RP7 based on the common occurrence of the primary index species Bekoma bidartensis. As at Sites U1406–U1408, Zone RP7 in Hole U1409A includes earliest Eocene indicator species Podocyrtis papalis, Theocorys? phyzella, and Theocorys? aff. phyzella (sensu Sanfilippo and Blome, 2001). However, nannofossils indicate that this interval is of Paleocene age. Sample 21X-CC (162.71 mbsf) is assigned to Subzone RP6c based on the co-occurrence of the primary index species for Zone RP6, Bekoma campechensis, and Buryella pentadica, which has its base in Subzone RP6c.

Planktonic foraminifers

Core catchers and additional samples from Hole U1409A working section halves were examined. The samples contain diverse and well-preserved assemblages of planktonic foraminifers from Pleistocene through Paleocene age. Depth positions and age estimates of identified biostratigraphic marker events are shown in Table T8. The stratigraphic distribution of planktonic foraminifers is shown in Table T9.

The uppermost Samples 342-U1409A-1H-CC to 2H-CC (0.97–10.61 mbsf) contain Globorotalia truncatulinoides and Globorotalia inflata, indicative of Pleistocene age. Sample 3H-CC (20.56 mbsf) contains a well-preserved but low-diversity assemblage of G. inflata, Neogloboquadrina dutertrei, and Neogloboquadrina pachyderma, suggesting an age of late Pliocene–Pleistocene. A poorly preserved Oligocene assemblage (mainly comprising Catapsydrax unicavus, Dentoglobigerina tapuriensis, Globorotaloides suteri, Subbotina angiporoides, and Subbotina corpulenta) was recovered from Samples 4H-2, 110–112 cm, through 4H-CC (22.7–30.03 mbsf). Poorly preserved planktonic foraminifers of middle Eocene age are found in Samples 5H-3, 25–27 cm, through 5H-3, 65–67 cm (32.85–33.25 mbsf). The presence of the marker species Acarinina bullbrooki, Globigerinathelka index, and Globigerinatheka mexicana suggests that these poorly preserved assemblages are from middle Eocene Zones E11–E14.

Well-preserved and taxonomically diverse planktonic foraminifers of middle Eocene age are found in Samples 5H-3, 110–112 cm, through 13H-CC (33.7–115.41 mbsf). The uppermost portion of this middle Eocene sequence is assigned to Zone E10 based on the co-occurrence of the marker species Guembelitrioides nuttalli, Morozovelloides lehneri, and G. index. The top of the morphologically distinctive Morozovella aragonensis marks the base of Zone E10 in Sample 7H-CC (58.5 mbsf).

The base of Globigerinatheka kugleri occurs in Sample 8H-5, 119–121 cm (65.29 mbsf), and is indicative of the base of Zone E9. The base of G. nuttalli in Sample 12H-CC (105.91 mbsf) indicates the base of Zone E8, whereas the base of Turborotalia frontosa in Sample 13H-CC (115.41 mbsf) indicates the base of Subzone E7b.

Samples 14H-2, 110–112 cm, through 14H-6, 110–112 cm (117.7–123.7 mbsf), contain planktonic foraminifers ranging from Subzone E7a to Zone E3. The base of Acarinina cuneicamerata in Sample 14H-6, 110–112 cm (123.7 mbsf), indicates the base of Subzone E7a, whereas the top of Morozovella subbotinae in Sample 14H-CC (124.86 mbsf) marks the base of the comparatively short Zone E6. The base of M. aragonensis in Sample 18H-3, 87–89 cm (135.47 mbsf), indicates the base of Zone E5, and the coincident bases of Morozovella formosa and Morozovella lensiformis in Sample 19H-CC (141.2 mbsf) mark the base of Zone E4.

Zone E3 extends from Sample 20X-1, 55–56 cm (151.35 mbsf), to 20X-3, 36–38 cm (154.16 mbsf). Directly beneath this interval lies a silicified interval representing the PETM interval. In Sample 20X-3, 100–101 cm (154.8 mbsf), we find the PETM “excursion taxon” Acarinina africana. Below this level, the uppermost Paleocene Zone P5 contains moderate to poorly preserved foraminifers downhole to Sample 20X-4, 29–30 cm (155.59 mbsf). In contrast, Subzone P4c contains well-preserved foraminifers and spans Samples 20X-5, 114–115 cm, through 21X-1, 133–134 cm (157.94–161.73 mbsf), as defined by the top of Globanomalina pseudomenardii and the base of Acarinina soldadoensis, respectively. We recognize Subzone P4b in Samples 21X-CC through 22X-2, 100–102 cm (162.68–172.5 mbsf), based on the presence of a well-preserved assemblage including Morozovella velascoensis, Morozovella occlusa, Morozovella pasionensis, Morozovella apanthesma, G. pseudomenardii, Acarinina subsphaerica, and Acarinina mckannai. The base of Subzone P4a is identified in Sample 22X-4, 100–102 cm (175.5 mbsf), by the base of G. pseudomenardii. Zone P3 spans from Sample 22X-CC to 25X-CC (179.32–197.75 mbsf), with its base marked by the base of Morozovella angulata.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in core catcher samples from Hole U1409A. Additional working section-half samples taken from Cores 342-U1409A-4H through 24X were examined for preservation and relative abundance of benthic foraminifers. Benthic foraminifers at this site are predominantly rare (the “present” category) relative to total sediment particles >150 µm in the Eocene and Paleocene and more abundant in the Oligocene (Fig. F22; Tables T10, T11).

Preservation of benthic foraminifer tests is generally good to very good in the Eocene to upper Paleocene, but the Oligocene and lower Paleocene successions contain poorly to moderately preserved benthic foraminifers (Fig. F22).

The well-preserved Pleistocene fauna of Samples 342-U1409A-1H-CC (0.97 mbsf) and 2H-CC (10.61 mbsf) is dominated by Pullenia bulloides, Pyrgo sp., Uvigerina peregrina, and Uvigerina senticosa. Sample 3H-CC (20.56 mbsf) is barren.

The lower Oligocene benthic foraminifer assemblage (Sample 4H-CC; 30.03 mbsf) is characterized by high-productivity fauna, with Cassidulina subglobosa and Stilostomella subspinosa being the dominant species.

Samples 5H-CC through 19X-CC (38.66–141.20 mbsf) show typical lower to middle Eocene fauna dominated by calcareous taxa. Abundant calcareous taxa include Alabamina dissonata, Bulimina sp., Chrysalogonium sp., Cibicidoides subspiratus, Dentalina sp., Nuttallides truempyi, Oridorsalis umbonatus, Pleurostomella acuta, and stilostomellids (Stilostomella gracillima, Stilostomella lepidula, and S. subspinosa). C. subglobosa is abundant in the middle Eocene part of the succession only (Samples 5H-CC through 10H-CC; 38.66–86.86 mbsf). Overall, the Eocene assemblages described above suggest a normal deepwater environment. An exception is Sample 16H-CC (126.91 mbsf), which contains a benthic foraminiferal assemblage dominated by the infaunal taxa Bigenerina sp. and Stilostomella sp. and abundant N. truempyi but lacks other epifaunal species (Fig. F23.

Samples 20X-CC through 26X-CC (160.56–200.00 mbsf) are represented by Paleocene taxa characterized by Bulimina sp., Dentalina sp., Gavelinella beccariiformis, Lenticulina whitei, Pullenia coryelli, and S. subspinosa. Agglutinated taxa are mainly represented by Dorothia trochoides, Gaudryina pyramidata, and Spiroplectammina spectabilis. In general, preservation and abundance of Paleocene benthic foraminifers decreases downhole. Sample 24X-CC (196.97 mbsf) is barren of benthic foraminifers.

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